The use of hydrogen as a convenient energy carrier and storage medium is increasingly gaining in interest, hydrogen being well recognized as a non-polluting, universally applicable fuel of high caloric value. Moreover, hydrogen may be considered to be available in unlimited quantities, but its general acceptance is conditional on the development of processes for its production. The fuel crises that erupted in 1973 is bound to accelerate the search for new and more economic processes for hydrogen production.
Many uses of hydrogen as fuel have been suggested, and some of them are already applied successfully. Thus, for instance, an improvement of 80% has been observed in the thermal efficiency of an internal combustion engine installed in a motor car, when it was converted for operation on hydrogen.
The concept of a "hydrogen heat pump", by which heat is alternatively absorbed and given off, is one device utilizing hydrogen as energy carrier and storage medium. In that type of hydrogen pump, the working medium is transferred back and forth between two metal hydride systems, the process involving hydrogen absorption and desorption. Broadly speaking, there are available both high-temperature hydrides, working in relatively high temperature ranges, and low-temperature hydrides for low-temperature uses. The hydrogen circulates in a closed circuit and is thus not consumed.
As is well known, metal hydride containers cool down during the desorption of hydrogen, and the utilization of this property for refrigeration has been suggested. This suggestion has however not yet been put to practical use because of certain drawbacks, of which the following two are especially noteworthy:
(1) Insufficient cooling rate, and PA1 (2) low cooling capacity per unit weight.
In a report published by Argonne National Laboratory (ANL-77-39, June 1977) entitled HYCSOS, a chemical heat pump and energy conversion system based on the hydride system CaNi.sub.5 -LaNi.sub.5 is proposed also as space conditioner and for refrigeration applications. The report, however, includes the qualification that the theoretical maximum of work to be drived in one energy conversion cycle, which represents the maximum extractable quantity of heat, is low. This is because the hydride, which is of the low-temperature variety, is the critical component of this particular type of air-conditioner, in which the cycle rate is determined by the rate of the hydrogen desorption from the hydride which is the slowest part of the process. With the aid of heat transfer calculations and actual experimental results it has been established that the desorption rate is, in turn, determined by the heat transfer rate because of the endothermic nature of the chemical reaction involved. This is true as long as the rate of the chemical reaction is higher than the rate of heat transfer needed for the reaction.